1. Field of the Invention
This invention relates to a tubular film process for a thermoplastic resin, and more particularly to improvements in a process of cooling molten resin extruded from an extrusion die.
2. Description of the Prior Art
In forming a blown film, in order to effect a high speed forming (a high extrusion output) to improve the productivity and to obtain a high quality film as well, it is necessary to cool a molten resin satisfactorily.
Heretofore, the most common blown film forming process has been such an arrangement that only one air ring to discharge cooling air in a direction of extrusion of the resin is disposed on an extrusion die and cooling air is blown out onto the outer peripheral surface of the molten resin, which has been extruded in a tubular form, by use of this air ring.
However, in the forming process of the type described, a hot air stream heated to a high temperature by heat exchange with the molten resin of a high temperature immediately after the resin has been extruded from the extrusion die accompanies the outer peripheral surface along the flow of the resin for a long period of time, thereby preventing a satisfactory cooling effect to the tubular resin. Moreover, if a discharge flow rate of the cooling air is increased to enhance the cooling effect, then the forming stability is hindered to a great extent, and hence, there has naturally been a limit imposed on the improvements in cooling effect by the process of the type described.
Now, there have been proposed a process wherein two air rings are provided for discharging cooling air in a direction of extrusion of the resin in two stages in the vertical direction (Japanese Patent "Kokai"(Laid-Open) No. 146764/78 and Japanese Patent "Kokoku"(Post-Exam. Publn.) No. 14295/82) and another process wherein a vacuum attracting mechanism is provided between the aforesaid air rings disposed in two stages, and a hot air stream of high temperature heated by the outer peripheral portion of the tubular resin is removed from the outer peripheral portion of the resin by means of this vacuum attracting mechanism (Japanese Patent "Kokai"(Laid-Open) No. 29370/79).
However, in the processes described above, the mechanisms are complicated, particularly, in the case of providing the vacuum attracting mechanism, a large-scale installation is needed, the operational control is difficult to perform, and the stable forming at high speed is not easily performed. Furthermore, except the above-described processes, for example, there has been known a process of cooling the tubular resin with water from outside or inside, however, similarly to the above, the installation has been very complicated and operational control has been very difficult to perform.